This invention relates in general to the application of foam coatings suitable for thermal insulation, and more specifically to the formation of low density foam coatings of uniform density and thickness.
Highly efficient thermal insulation materials are becoming increasingly necessary for applications such as cryogenic fuel tanks in high energy space launch vehicles, liquefied natural gas storage tanks, ships for transporting liquefied natural gas, and other similar applications. The insulation must be highly efficient, light in weight, and reasonably inexpensive to apply and maintain over very large structures.
Many insulation systems are in use today. While many, such as asbestos packing, glass fiber batts, etc., are useful when the temperature difference within and without an insulated structure is not great, most of these are unacceptable for use with cryogenic tanks where this temperature difference is in hundreds of degrees. Insufficient insulation results in, for example, an undesirable waste of cryogenic liquids due to inward heat transfer and the resulting boil-off and venting of part of the liquid.
A number of "super insulations" have been developed for applications requiring very high thermal insulation efficiency. These include multi-layer metalized plastic sheets, low density foam, etc. These insulation arrangements, while generally effective, tend to be complex, cumbersome and difficult to install and maintain on large structures. Foam often must be cut to shape from large blocks and installed piece by piece. Attempts to form foam in place directly on structures has had only limited success due to difficulties in forming insitu foam layers of uniform low density and thickness. Spraying a self-rising foam directly on a structure tends to produce layers of very uneven thickness with an undesirable rind or skin on the outside surface which are difficult and expensive to machine to a uniform thickness. Applying foam within a closed mold in contact with the structure tends to produce layers of higher density than desired, and of uneven density, since the foam cannot fully expand. Cutting holes in the mold to permit excess foam to extrude therefrom has not been successful since the resulting layer has uneven density, lower near the holes and higher elsewhere. Also, the backpressure resulting from the force needed to push excess foam through the holes tends to cause the layer to have a higher than desirable average density. The surface of the foam in contact with the mold walls tends to form an undesirable rind. Because of these difficulties in producing uniform foam insulation layers, many cryogenic applications must use the much more costly and complex multi-layer insulation systems.
Many of the problems of non-uniformity and expense are aggravated where a very large structure must be coated by covering many small contiguous areas one at a time. While devices have been designed for continuously producing long panels or slabs of foamed materials, these produce foam of higher density than is required for insulation purposes, and the foam tends to be uneven in density.
Thus, there is a continuing need for improved high efficiency insulation coatings, especially for the insulation of very large structures containing materials at cryogenic temperatures.